The present invention relates to rotary fluid coupling devices, and more particularly to such devices wherein heat is generated as a result of the torque transmission, and the ability of the coupling device to respond to changes in temperature is an important performance criterion.
Rotary fluid coupling devices of the type which may benefit from the use of the present invention have found many uses, one of the most common of which is to drive the cooling fan associated with the radiator of a vehicle engine. Such coupling devices are frequently referred to as "viscous fan drives" because such couplings utilize a high-viscosity fluid to transmit torque, by means of viscous shear drag, from an input coupling member (clutch) to an output coupling member (housing), to which is bolted the cooling fan.
More specifically, the present invention is especially advantageous when used on a relatively high-torque viscous fan drive, i.e., a fan drive which is capable of transmitting to the cooling fan in the range of about 2 horsepower to about 12 horsepower, although it should be understood that the invention is not so limited. Typically, such high-torque or high-horsepower fan drives include an output coupling assembly comprising a cast aluminum housing and a die cast aluminum cover. The input coupling member and the die cast cover normally include a plurality of interdigitated lands and grooves which define the shear space. When this shear space is filled with viscous fluid, torque is transmitted from the input coupling member to the output coupling assembly, in response to rotation of the input coupling member.
During torque transmission, substantial heat is generated as a result of the shearing of the viscous fluid, and the cast cover is the primary heat dissipating element of the device. Therefore, it has been conventional practice in such viscous fan drives to have a plurality of cooling fins cast integrally with the cover. An example of a fan drive to which the present invention may be advantageously applied is shown in U.S. Pat. No. 4,678,070, assigned to the assignee of the present invention and incorporated herein by reference. In such fan drives, the cast cover defines a raised, annular, reservoir-defining portion, and disposed within the recess defined by this annular portion is the bimetal coil which provides temperature responsive operation of the valving within the coupling device.
The air in the recess where the bimetal coil is located is relatively stagnant, because the coil and the recess are located on the axis of rotation of the coupling device, which is also the center line of the natural stream of air flow past the device. Changes in the temperature of the water in the radiator, resulting in changes in the temperature of the air flowing through the radiator, indicate changes in the need for fan drive operation. Heat is transferred from the flowing air to the bimetal coil by forced convection, but with relatively little air flow in the region of the coil, the fan drive may not respond as quickly as desired to changes in coolant temperature. More specifically, when the fan drive has been disengaged, and a situation occurs which demands relatively greater cooling, the temperature of the coolant rises rapidly, as does the temperature of the air being drawn through the radiator. However, the bimetal coil does not heat up as rapidly because of the relatively stagnant air surrounding the coil, and the engine may overheat before the coil has been heated sufficiently to cause the fan drive to engage.
If the temperature responsiveness of a viscous fan drive can be substantially improved, it would then be possible to set the calibration at a higher temperature, i.e., the fan drive would engage at a higher temperature. A higher calibration temperature would mean that the tan clutch would engage less frequently thus reducing the engine horsepower consumed by the fan drive, but perhaps more importantly to the customer, reducing the amount of time (frequency of engagement) that the fan drive spends in the engaged condition. Keeping the fan drive disengaged more of the time reduces the amount of undesirable fan noise.